The present invention relates to an aluminum alloy-made forged scroll part for a scroll compressor employed mainly in an air conditioner and to a process for producing the part.
In recent years, scroll compressors have become of great interest as air conditioner compressors, because, for one reason, such a scroll compressor contains a small number of parts and is driven silently. The scroll compressor includes a fixed scroll having a spiral wrap portion 11 provided on a flange portion 12 as shown in FIG. 2, and an orbiting scroll having a spiral wrap portion whose shape is similar to that of the portion 11, the spiral wrap portion of the orbiting scroll being driven for orbital movement such that these spiral wrap portions face each other in a fitted state.
In many cases, a fixed or orbiting scroll (hereinafter referred to simply as a xe2x80x9cscrollxe2x80x9d) is produced from aluminum alloy in order to reduce the weight of a resultant compressor. The scroll is produced through, for example, casting or forging. In order to provide the scroll with strength and reliability, forging is advantageously carried out for producing the scroll. Since the scroll has a complicated shape, it must be produced through hot forging.
FIG. 3 shows a conventional process for production an aluminum alloy scroll part through forging.
First, an aluminum alloy prepared by mixing alloy components is melted and then cast through continuous casting into a billet (BL) for extrusion having a diameter of 200 mm or more. After the inside of the BL is homogenized through heat treatment, the BL is cut into pieces such that they have an identical length so as to provide round bars each having a predetermined diameter, and each piece is subjected to extrusion to thereby form a round bar (extruded round bar).
Usually, the diameter of the extruded round bar is almost equal to the outer diameter of a forged part. The round bar is cut into pieces, and each piece is employed as a stock material for forging. As will be described below, in order to facilitate production of a scroll part, before forging of the stock material the cut piece may be pre-shaped, if necessary, through forging or machining into a piece having a shape similar to that of the scroll part, so as to employ the pre-shaped piece as a stock material for forging.
The stock material is forged into a scroll part usually through hot forging. In order to provide the forged part with strength, after forging, the part is usually subjected to solution (quenching) and aging heat treatment.
In order to enhance precision in the size of the forged part, a portion of the surface of the part is then subjected to machining, if necessary.
FIG. 4 is a schematic cross-sectional view showing a conventional scroll-forging process. A workpiece 4 placed in a die 2 is pressed downward with a punch 1 to thereby form the wrap portion 11. Usually, the distance that the punch 1 moves is determined to be consistent in order to make the thickness of a flange portion 12 of the scroll consistent.
JP-A-SHO 54-159712, 59-61542 and 62-89545 disclose a process for forging an aluminum alloy-made scroll, in which, in order to precisely forge a workpiece into a scroll wrap, the workpiece is subjected to forging or machining in advance so as to provide the piece with a preliminary shape, and the workpiece is then forged into the scroll wrap. The reason why such a preliminary process is carried out is that since the wrap portion 11 has a spiral shape and large height and is connected to the flange portion 12, when a workpiece is forged into a scroll as shown in FIG. 4, a wrap portion having a uniform height is difficult to form. Therefore, a workpiece having an intermediate shape is formed in advance. The process can provide a produced scroll with a shape with some degree of precision. However, the process requires designing of an intermediate shape which matches the final shape of the scroll, and preparation of a forging die employed for intermediate processing. Consequently, the process includes complicated steps and involves high costs, presenting difficulty in practice.
JP-A-SHO 60-102243 and JP-A-HEI 06-23474, among other publications, disclose a back-pressure forging process in which a workpiece prepared only by cutting a round bar is employed without being subjected to pre-processing before forging and, during forging of the workpiece, a load is applied to the end portion of a scroll wrap 11 in a direction opposite to the forging direction in order to control material flow so as to realize a uniform flow into a wrap-shaped mold and to reduce variation in the height of the scroll wrap 11. According to the process, by using a workpiece prepared only by cutting a round bar, a scroll in which variation in the height of a wrap portion 11 is reduced can be produced at low cost with high productivity.
To be specific, the back-pressure forging process for a scroll is schematically shown in the cross-sectional views of FIGS. 5 and 6. A workpiece 4 is pressed downward with a punch 1 and forged into a wrap formation space 2a of a die 2 while the knockouts are retracted to thereby form a wrap 11. During the forging, a load lower than a punch pressure is applied as a back pressure through the wrap formation space 2a by means of knock pins 7 and knockouts 6 to the end of the wrap in the direction opposite to that of the forging (FIG. 5). As a result, a scroll part 5 comprising a flange portion 12 with a predetermined thickness L1 and the wrap 11 with a uniform height L2 depending vertically from the flange portion can be formed as shown in FIG. 7.
The back-pressure forging process exerts, to some extent, the effect for making the overall height of a spiral wrap of a forged scroll part uniform.
Although variation in the height of a wrap of a scroll can be regulated to some extent according to the back-pressure forging process, wrap height varies between individual scrolls unless the thickness of individual cut materials, i.e. the weight of individual workpieces, is strictly controlled when cutting the round bar. Therefore, a margin for machining of the end of a wrap must be controlled in every forged part at a post-processing step. Alternatively, in consideration of different wrap heights among scroll products, slightly large-sized scrolls must be forged to provide scrolls with a large margin for machining at a post-processing step. This results in low yield.
In the back-pressure forging process, when a workpiece is forged into a scroll, the thickness L1 of the flange portion 12 is controlled by a stroke of the punch 1, and the remaining portion of the workpiece is forged into a wrap portion. Therefore, difference in the volume of the workpieces before forging is reflected in difference in the height L2 of the wrap portions.
Conventionally, in order to smoothly carry out forging of a workpiece without production loss, the workpiece is prepared by cutting a round bar material having a diameter nearly equal to the outer diameter of a flange portion that will become the maximum outer diameter of a forged scroll. Therefore, variation in the thickness of the cut material is reflected in variation in the volume of the workpiece, i.e. variation in the height of a wrap portion of the scroll.
The horizontal cross-sectional area of a wrap portion is about ⅓ to ⅕ that of a workpiece. Accordingly, the variation in the cut length of the workpiece results in a variation in the height of the wrap portion that is 3 to 5 times the variation in the cut length. Therefore, a margin of the end of the wrap for machining in a post-processing step cannot be reduced because the margin has to include the variation in height. For this reason, a plural number of machining steps are required, resulting in failure to reduce the manhour for machining of scrolls and enhance the material-based yield.
In consideration of conditions under which scrolls are used, an aluminum alloy material containing a large amount of silicon is employed for producing a scroll in order to enhance strength and wear resistance of the scroll. Since the material is hard, a blade for cutting the material is easily worn. Therefore, compared with a conventionally used alloy, variation in the thickness of the aluminum alloy material increases during cutting, greatly affecting variation in wrap height between individual forged scrolls.
In addition, a forging process that can forge a scroll part into a shape approximating to a product shape as well as the wrap height forging has recently been desired. The formation of concave portions in the surface of a flange provided with a wrap as shown in FIG. 8 is accompanied with metal flow toward the wrap and metal interference that result in sand or slag inclusion or other forging defects particularly when forging under the condition not utilizing a back pressure. Therefore, the concave portions cannot be obtained using a one-step forging process, and a plural-step forging process has been adopted in general. Actually, however, a machining process rather than the plural-step forging process has been selected in the formation of concave portions from the standpoint of labor and cost. This incurs machining process cost.
As described above, an aluminum alloy material is employed for producing a scroll in order to reduce the weight of the scroll. In consideration of high strength, high wear resistance and a balance of these to processability, Alxe2x80x94Si alloy materials among a variety of aluminum alloy materials have mainly been developed. When the characteristics of the material are regulated, fine Si particles are uniformly dispersed in an aluminum base in order to impart wear resistance to the material. Development of alloy materials other than Alxe2x80x94Si alloy materials has encountered difficulty to date, and thus such other alloy materials are not employed in practice, and basically modifications of Alxe2x80x94Si alloy materials are carried out.
In an Alxe2x80x94Si alloy material, crystallization of Si particles is indispensable to enhancement of wear resistance of the material. However, crystallization of coarse primary Si crystals having a size of tens of pm or more induces wear of a blade during machining, causing a product to have a rough machined surface. In addition, when such coarse primary Si crystals segregate at a portion of a scroll subjected to high stress, fatigue breakage initiates at that portion when the scroll is employed, greatly impairing reliability of the scroll. Furthermore, as described above, when such an Alxe2x80x94Si alloy material is cut, wear of a blade is accelerated, and thus variation in the thickness of the material increases during cutting.
As described above, in a conventional production process, such an aluminum alloy material is formed, through cutting, into an extrusion round bar material. In order to form the round bar material, the alloy material is usually cast, through continuous casting, into a billet having a relatively large diameter (200 mm or more). Therefore, the billet is solidified slowly during casting, and thus crystallization of coarse primary Si crystals having a size of 100 xcexcm or more tends to occur, and control of distribution of Si particles in the billet is difficult. Furthermore, when the coarse Si crystals are crystallized in the material as described above, variation in the thickness of the billet may occur during cutting. In addition, primary Si crystals remain in a forged scroll product as a large, hard impurity, and the crystals may cause problems in machining of the forged scroll and reduction in strength thereof.
One object of the present invention is to provide an aluminum alloy-forged scroll part that enables reduction of a variation in wrap height of a scroll part as well as reduction of a variation in height of the wrap portion of a forged product and a process for producing the scroll part.
Another object of the present invention is to provide an aluminum alloy-formed scroll part that enables reduction in a margin for machining in post-processing and suppression of occurrence of coarse primary Si crystals which would cause wear of a blade during machining and reduction in strength of the forged scroll part.
The present invention provides an aluminum alloy-forged scroll part characterized in that it is produced from an aluminum alloy material comprising 8.0-12.5 mass % of Si, 1.0-5.0 mass % of Cu and 0.2-1.3 mass % of Mg, and that the scroll part contains Si particles having a size of less than 15 xcexcz and a mean size of 3 xcexcm or less. The Si particles include primary Si particles and eutectic Si particles.
The present invention further provides a process for producing an aluminum alloy-forged scroll part comprising a step of casting an aluminum alloy that comprises 8.0-12.5 mass % of Si, 1.0-5.0 mass % of Cu and 0.2-1.3 mass % of Mg into a round bar having a diameter of 130 mm or less, preferably 85 mm or less, a step of cutting the aluminum alloy round bar into a stock material for forging, a step of subjecting the stock material to upsetting at an upsetting ratio of 20-70% to form a pre-shaped product that is a workpiece, and a forging step of applying pressure onto the workpiece with a punch at a temperature of 300-450xc2x0 C. to form a scroll wrap in a direction of applying the punch pressure, and wherein the forging step includes a step of applying a back pressure that is lower than the punch pressure to an end of the press-formed scroll wrap in a direction opposite to the punch pressure applying direction.
The aluminum alloy may further comprise 2.0 mass % or less of Ni and/or 0.5 mass % or less of one or more species selected from among Sr, Ca, Na and Sb.
The back pressure may be a constant pressure of 80-240 N/mm2 or comprise an initial pressure of 80-240 N/mm2, a pressure gradually reduced from the initiation of wrap formation and an end pressure of 40 to 120 N/mm2.
The stock material subjected to upsetting may be subjected beforehand to homogenization heat treatment at 480-520xc2x0 C. for 0.5-4 hours and/or to surface peeling.
The surface of the workpiece subjected to forging may be coated with a lubrication film.
The forged part may be subjected to solution heat treatment (quenching) and aging treatment (quenching, aging and hardening treatment).
Conventionally, when aluminum alloy is to be cast as a billet for ordinary extrusion, the cast billet ordinarily has a large diameter of 200 mm or more. For this reason, the billet is cooled slowly and solidified moderately. When the Si content exceeds 10%, therefore, crystallization of coarse Si crystals having a size of around 100 xcexcm as primary crystals tends to occur. Even in a small-diameter bar obtained by extrusion of the billet, the crystals tend to remain. While the primary Si crystals are easy to segregate at the center part of a billet that is cooled slowly in particular, they exist at random over the entire lateral cross section of the billet when the Si content approximates to 12%.
In the present invention, however, when forging aluminum alloy into a round bar, the diameter of the forged bar is set to 130 mm or less, as described above. As a result, its cooling speed is considerably higher than that of a billet having a diameter of 200 mm to the effect that its solidifying speed is high. This enables eutectic Si crystals to be made smaller to suppress occurrence of coarse primary Si crystals.
By making the diameter of the circular bar small as described above, occurrence of course primary Si crystals can be suppressed. Therefore, the problem of wear of a blade during machining that would cause deterioration of the quality and reduction in strength of a product can be solved. In addition, the small diameter of the circular bar can reduce a margin for machining of post-processing, resulting in an economical advantage.
Further, the present invention has two features, one of which is to carry out forging using a back pressure two to four times the general back pressure for the purpose of promoting preferential formation of a flange portion. The other feature is to control the formation process by varying the back pressure stepwise in accordance with the forging process, while it is general to apply a constant pressure at the forging step in the back-pressure forging. These features enable reduction of a variation in height of a wrap in a scroll part and in every scroll part being forged.